In general, optical disk substrates are formed by injection molding. The optical disk substrate is made of thermoplastic resin. FIG. 16 shows an example of a mold used for injection molding. In FIG. 16, numeral 101 denotes a stationary mold, while numeral 102 denotes a movable mold.
A sprue bush 103 serving as an entrance for melted resin is provided at the center of the stationary mold 101. A stamper holder 104 is located between the sprue bush 103 and a stationary mirror surface plate 105. On the stationary mirror surface plate 105, there is located a stamper 106 formed with pits and lands bearing information. The stamper 106 is fixed on the stationary mirror surface plate 105 by means of the stamper holder 104 and a peripheral ring 107. The stationary mirror surface plate 105 is fixed on a stationary base plate 108.
A movable mold 102 is provided with an ejector pin 109, a cutter punch 110, an ejector 111, a movable side fixing bush 112 and a movable mirror surface plate 113 which are arranged in the recited order from the center of the mold. The cutter punch 110 has a function of forming an inner hole in the optical disk substrate by projecting through the optical disk substrate. The ejector 111 has a function of releasing the molded disk from the movable mold 102. The ejector pin 109 has a function of pushing out the sprue portion that has been removed from the optical disk substrate to form the inner hole. The movable side fixing bush 112 has a function of preventing the ejector 111 from directly contacting the movable mirror surface plate 113 and preventing wear of the movable mirror surface plate 113. The movable mirror surface plate 113 is fixed on a movable base plate 114.
A stationary abutting ring 115 is located at the extreme periphery of a stationary mold 101, while a movable abutting ring 116 is located at the extreme periphery of the movable mold 102. The fitting of the movable abutting ring 116 with the stationary abutting ring 115 made the molds 101 and 102 concentric with each other.
FIGS. 17A and 17B show a manner of forming the inner hole. FIG. 17A represents a condition in which melted resin is injected through the sprue hole into the mold with the stationary mold 101 and the movable mold 102 being closed. Next, while the resin is in the melted condition, the cutter punch 110 is caused to fit the sprue bush 103 with the position of the cutter punch relative to the ejector pin 109 being kept unchanged, as shown in FIG. 17B. By this fitting, the inner hole is to be formed in the optical disk substrate.
On the other hand, Patent Document 1 discloses a way to form a disk with an inner hole in which way a member of the stationary mold is slid to project toward a member of the movable mold so that the sprue bush fits the ejector.
Further, Patent Document 2 discloses making the cutter punch of the movable mold concave, and making the sprue bush convex so that the cutter punch fits the sprue bush.
Alternatively, it has been also proposed to form the disk with the inner hole without the fitting of members. Patent Document 3, for example, discloses molding an optical disk substrate with the sprue connected by a thin film connecting portion. After the resin has been solidified, the connecting portion is cut by projecting the ejector sleeve upon releasing of the optical disk substrate. Thus, the sprue is removed from the optical disk substrate.
According to the ways shown in Patent Documents 1 and 2 in which a couple of members fit each other while the resin is in melted condition, a flash would be caused at the fitting clearance between the couple of members by the melted resin thrusting into the clearance. In this case, depending on which of the male member and female member for forming the hole is provided in which of the stationary mold and the movable mold, the flash is formed on either end of the inner hole as shown in FIG. 18A or FIG. 18B. In the case of Patent Document 3, on the other hand, the flash is caused in the midway of the inner hole as shown in FIG. 18C.
As a way to remove the flash, a process of cutting it by a cutter is disclosed in Patent Document 4, another way of melting it down by a burner is disclosed in Patent Document 5, and still another way of decomposing it by ultraviolet light is disclosed in Patent Document 6.
Patent Document 7 also discloses a technology relating to the flash although it is not for forming an optical disk substrate. Patent Document 7 discloses a method for producing a plastic molding product wherein a high pressure of air introduced into the cavity through a mold (e.g., upper mold) which forms the back surface of the product to depress the resin mold product against the lower mold, thereby preventing generation of sink marks on its outer surface. Patent Document 7 teaches to define the dimension of the clearance for introducing the high pressure of air in order to prevent the melted resin from thrusting into the clearance to thereby preventing generation of the flash.
[Patent Document 1] Japanese Patent No. 1944425
[Patent Document 2] Japanese patent application laid open No. 2002-240101
[Patent Document 3] Japanese Patent No. 2071462
[Patent Document 4] Japanese patent application laid open No. Hei4-235006
[Patent Document 5] Japanese patent application laid open No. Sho59-196212
[Patent Document 6] Japanese patent application laid open No. Hei6-99581
[Patent Document 7] Japanese patent application laid open No. 2005-28731
Even if a step of removing flash such as shown in any of Patent Documents 4, 5 and 6 is provided, a problem still remains in that a flash is incompletely removed to cause a remaining flash leaves from the disk during the delivery thereof to stick to the surface of the disk as undesirable dirt. In the case that the plastic mold product is an optical disk substrate, especially, the undesirable dirt would deteriorate the quality of information thereon. Further, the flash may be sandwiched in between optical disk substrates when they are adhered to each other, resulting in inaccuracy in the thickness of the completed optical disk product. Further, the occurrence of flash varies along the circumference of the inner hole. Therefore, when an optical disk produced with the optical disk substrates suffered from the flash in the vicinity of the inner hole is set in a disk drive, a remarkable eccentricity occurs.
The necessity of removing the flash increases the number of steps for production. Further, an inaccurate removal of the flash would possibly influence on the diameter of the inner hole.
It may be possible to narrow the clearance between the components of the mold that cause the flash at issue, for preventing the resin from thrusting into the clearance. This approach, however, poses another problem in that it is difficult to carry out maintenance of the mold by deassempling and reassembling the mold because the components of the mold are fitted with each other tightly. Further, if a component of the mold causing the flash is of a type sliding on another component, wear of components of the mold would be remarkably increased.